snrp 101: fond du lac wwtp lab experiences · snrp 101: fond du lac wwtp lab experiences autumn...
TRANSCRIPT
SNRP 101: Fond du Lac WWTP Lab Experiences
Autumn Fisher1*
, Jeremy Cramer1, Eric Lynne
2
1 Wastewater Treatment Plant, City of Fond du Lac, Wisconsin
2 Donohue & Associates, Inc., Sheboygan, Wisconsin
* Email: [email protected]
ABSTRACT
The City of Fond du Lac, Wisconsin Regional Wastewater Treatment Plant (FDL
WWTP) has a proposed phosphorus water quality based effluent limit (WQBEL)
of 0.04 milligrams per liter (mg/L) effective January 1, 2022. Because of this
stringent limit, the plant laboratory staff has explored a multitude of phosphorus
species with a primary focus on soluble non-reactive phosphorus (sNRP). FDL
currently experiences a sNRP average of 0.08 mg/L and has become increasingly
interested in how this sNRP level may affect the ability to meet a future
phosphorus limit of 0.04 mg/L.
KEYWORDS: Phosphorus speciation, soluble non-reactive phosphorus (sNRP),
phosphorus removal, low phosphorus concentration
INTRODUCTION
The City of Fond du Lac, Wisconsin Regional Wastewater Treatment Facility (FDL WWTP)
serves the population of not only the City of Fond du Lac which is approximately 43,000 people
but also the population of an Outlying Sewer Group which includes an additional 30,000 people
and has a design flow of 9.84 MGD. The facility utilizes the activated sludge process and
anaerobic digestion and typically uses aluminum sulfate for phosphorus removal.
Like many other wastewater treatment facilities in the state of Wisconsin, the FDL WWTP is
preparing to meet a much lower final effluent phosphorus limit. A predicated effluent limit of
0.04 mg/L has forced the facility to look in to multiple options and treatment methods to
determine the best course of action to meet the future limit. The options and treatment methods
piloted here include the following: CoMag™, full-scale biological phosphorus removal, full-
scale chemical phosphorus removal using cerium chloride (SorbX 100), Ovivo® TriSep membrane
filtration, ACTIFLO®,
Aqua-Aerobics AquaDisk
® followed by Aqua Ultrafiltration™, and even
a nutrient harvesting process. Over the course of the past several years, each pilot study that has
been conducted at the facility to determine if a 0.04 mg/L concentration is achievable has shown
that it will be difficult and costly meet the future limit. It has become apparent there is
something affecting the ability of most of these processes to achieve results below the 0.04 mg/L
concentration.
For each pilot study, the on-site laboratory staff has played a significant role in analyzing and
collecting data. In evaluation of phosphorus removal efficiencies of the pilots, sNRP presence
has been suspected to affect achievable levels of phosphorus removal. Due to this finding, the
laboratory staff has devoted a significant amount of time investigating this phenomenon,
including developing a standard operating procedure to analyze and properly measure sNRP and
validate the accuracy of all data obtained.
In addition to the pilot study findings and daily final effluent phosphorus speciation, it has been
noticed that some treatment processes may influence the removal levels of sNRP. For this
reason, the laboratory staff has also analyzed numerous samples that have been collected from
high strength waste haulers, industries, Outside Sewer Group, and normal influent wastewater
flows to determine levels of incoming sNRP. The reason much emphasis has been focused on
sNRP levels at FDL is the average sNRP value is around 0.08 mg/L. This may pose a serious
problem if the facility needs to meet a 0.04 mg/L limit.
Another reason sNRP may be an issue for the FDL WWTP is the fact that the facility will not
only have a concentration limit of 0.04 mg/L but will also have a mass limit. The mass limit will
be an important consideration because FDL is greatly impacted by inflow and infiltration and
during a high flow rain event, the facility may have difficulties meeting the mass limit. Overall,
it has been found that only certain processes seem to show good removal efficiencies and it may
require a combination of treatment processes/methods to remove the sNRP below 0.04 mg/L at a
significant cost to the rate-payers of the City. Ultimately, the level of sNRP in the FDL
wastewater may determine what treatment methods are utilized in the future at the facility. This
has given rise to the importance of knowing various sNRP data points and understanding
contributors.
METHODOLOGY
Phosphorus can be broken down into two primary forms, organic and inorganic. With this
distinction, further speciation can be conducted to determine the various phosphorus species such
as insoluble, polymerized, and non-reactive. Particulate phosphorus refers to any form of
phosphorus which is retained by a 0.45 µm pore size filter. Soluble/dissolved/filterable, which
can all be used interchangeably, refer to any form of phosphorus which passes through a filter
with a pore size of 0.45 µm. It should be noted that the filtrate that passes through a 0.45 µm
filter may also contain fine particles and colloids (Neethling, et al. 2007). sNRP can be
calculated by determining the difference between soluble total phosphorus (sTP) and soluble
reactive phosphorus (sRP). A majority of sNRP is soluble organic (sOP) and the remainder is
soluble polymerized (sPoly). Figure 1 shows a breakdown of how the various fractions were
analyzed and calculated.
Figure 1 - Phosphorus speciation flow chart and calculations.
The FDL laboratory used Standard Methods analytical methods to distinguish various
phosphorus fractions in their wastewater and other facility contributors. The FDL laboratory has
fine-tuned their technique and has determined that the dirtier a sample is, the longer the sample
takes to filter. Therefore, in some circumstances for analysis of sludge, industrial, hauler, or
plant influent samples it was necessary to pre-filter samples with a glass fiber 1.5 µm filter prior
to filtration with the 0.45 µm filter. Another helpful technique utilizes a benchtop centrifuge
followed by filtration of the sample.
Instrumentation used for the analysis of samples was a Thermo Scientific GENESYS 10
Spectrophometer with a MDL of 0.01 mg/L and a Seal AQ2 Discrete Analyzer with a MDL of
0.005 mg/L.
Careful consideration was given to contamination and the laboratory periodically analyzed
filtered blanks to ensure no background could be attributed to filtration technique and equipment.
RESULTS/DISCUSSION
The FDL laboratory has been analyzing their sNRP in excess of a year and a half and has
become well adept at sNRP analysis. The FDL WWTP currently experiences sNRP levels of
approximately 0.08 mg/L which is double the concentration of the 0.04 mg/L WQBEL. See
Figure 2.
Figure 2 – FDL WWTP sNRP average versus proposed WQBEL.
Fond du Lac relied on Neethling (2007) to provide general direction on sNRP analysis and
technique. However, because of the vast amount of data generated, it was important to the FDL
WWTP to not only generate data for research but also ensure the data being collected was
accurate. Therefore, the FDL laboratory conducted a comparison study with the Wisconsin State
Lab of Hygiene (WSLH) to confirm its laboratory results.
Overall, the FDL laboratory results when compared to the WSLH had a relative percent
difference (RPD) of less than 25%. The Wisconsin Department of Natural Resources (WiDNR)
typically recommends comparison results be within 10% but in the case of lower concentration
samples they allow for up to 25%. The FDL laboratory trended very closely with the WSLH for
sTP analysis (See Figure 3) but experienced RPDs of up to15% for comparison sRP results (See
Figure 4). It is speculated that the increased RPDs for sRP analyses was experienced due to the
fact that the WSLH performed the filtration step for sRP samples analyzed at their facility and
the FDL staff performed filtration for sRP samples analyzed in-house. The same phenomenon
was not experienced with the analysis of sTP samples because FDL submitted a filtered sample
to the WSLH for the sTP analysis. Differences in filtration equipment and analyst technique
likely contributed to the discrepancies in sRP data.
Figure 3 – FDL laboratory versus WSLH sTP results.
Figure 4 - FDL laboratory versus WSLH sRP results.
Further breakdown of phosphorus species can be conducted with the analysis of total phosphorus
(TP), sTP, acid hydrolyzable (tAHP), soluble acid hydrolyzable (sAHP), reactive phosphorus
(tRP), and sRP. With the analysis of these six phosphorus fractions, the FDL laboratory was
able to determine the average phosphorus fractions that can be attributed to things such as
organically bound and polymerized phosphorus in the plant influent versus the final effluent.
The results can be found in table 1.
Table 1 – FDL average influent and effluent phosphorus species concentrations.
Phosphorus Species Influent Effluent
A Total Phosphorus (TP) 4.635 0.814
B Total Soluble Phosphorus (sTP) 2.448 0.576
C Total Acid Hydrolyzable Phosphorus (tAHP) 3.497 0.697
D Total Soluble Acid Hydrolyzable Phosphorus (sAHP) 2.223 0.535
E Total Reactive Phosphorus (tRP) 2.787 0.601
F Total Soluble Reactive Phosphorus (sRP) 2.126 0.500
A-B Total Insoluble (pTP) 2.187 0.238
C-E Total Polymerized (tPoly) 0.710 0.095
D-F Soluble Polymerized (sPoly) 0.096 0.035
A-C Total Organically Bound (tOP) 1.138 0.117
B-D Soluble Organically Bound (sOP) 0.225 0.041
A-E Total Non-Reactive (tNRP) 1.849 0.212
B-F Soluble Non-Reactive (sNRP) 0.321 0.076
E-F Particulate Reactive P (pRP) 0.660 0.101
The FDL WWTP phosphorus was further evaluated for differences attributed to insoluble and
soluble reactive fractions. A breakdown of these phosphorus species can be found in Figure 5
below. When evaluated under these conditions, the phosphorus most often experienced at the
FDL WWTP is sRP with pTP and sNRP also present in varying degrees.
Figure 5 – FDL insoluble versus soluble speciation.
According to Neethling et al. (2007) phosphorus species vary from facility to facility depending
on treatment characteristics such as type of phosphorus removal methods (chemical, biological,
or filtration) as well as plant recycle flows and substrate acceptance policies. Therefore, FDL
felt it was necessary that in addition to daily final effluent sNRP determinations, plant influent
should also be analyzed for various fractions of phosphorus to determine facility removal
efficiencies. Figure 6 shows the FDL WWTP sNRP removal efficiency.
Figure 6 – FDL WWTP influent versus final effluent sNRP.
In addition to monitoring the facility effectiveness on removal of sNRP from the influent to the
final effluent, the data was scrutinized to determine any trends that may arise. When looking at
sNRP by day of the week, it became apparent that sNRP increased throughout the week with the
facility experiencing the highest average sNRP values on Wednesday and Thursday as can be
seen in Figure 7. The increase in sNRP throughout the week coincides with the operation of our
dewatering centrifuges and our centrate recycle stream and demonstrates the impact the side
stream has on our facility sNRP.
Figure 7 – FDL sNRP by day of the week.
As part of our facility study of feasible alternatives, the FDL WWTP has conducted a variety of
tertiary treatment pilot studies including CoMag™ and Actiflo® (ballast settled floc processes),
Ovivo® (ultrafiltration membrane), AquaAerobics
® (cloth disk followed by membrane filtration),
AirPrex™ (nutrient harvesting), and SorbX® (cerium chloride phosphorus removal chemical).
The FDL laboratory performed extensive sNRP analysis during the Ovivo, Aqua Aerobics, and
Actiflo pilot projects to determine removal capabilities. The graphs in Figure 7 show the
removal efficiencies of sNRP in blue, orange, and gray experienced during these pilots with a
table below indicating the pilot total phosphorus results.
Figure 7 – FDL pilot phosphorus removal results.
CONCLUSIONS
Analysis of the various fractions of phosphorus has provided the City of Fond du Lac valuable
information towards their future compliance options. Specifically, evaluations of sNRP have
been targeted to determine the feasibility of meeting future effluent requirements. Current
effluent sNRP concentrations exceed future WQBELs, therefore phosphorus reductions of all
fractions are necessary. Elevated sNRP was observed to increase with increased side stream
loadings. sNRP is linked to the presence of colloidal particles. This link is further supported by
the pilot testing data, which exhibits a trend of increased sNRP removal with increased colloidal
removal (coagulation and flocculation) upstream of the technology.
Moving forward, Fond du Lac hopes to acquire recommendations and/or comments from the
Wisconsin DNR in relation to sNRP and what it may mean for area TMDL studies and
phosphorus WQBELs. Future direction for Fond du Lac will likely include assessing changes in
sNRP under various treatment conditions such as centrate recycle changes and bio-P.
Additionally, we will continue to assess local industries, high-strength waste, and
septage/holding/portable waste. Fond du Lac would also like to conduct a bioavailability study
using algae and assessing phosphorus uptake.
ACKNOWLEDGEMENTS
The authors would like to thank the Fond du Lac laboratory staff for their extensive efforts in
sample handling and abundant laboratory analysis. Additionally, we would like to thank the
Wisconsin State Lab of Hygiene for their efforts during our eight week comparison study.
REFERENCES
Neethling, J.B., Benisch M., Clark, D., Fisher, D., and Gu, A. (2007) Phosphorus speciation
provides direction to produce 10 µg/L Proceeding of WEF/IWA Nutrient Removal
Conference, Baltimore.
Standard Methods for the Examination of Water and Wastewater 21st Edition (2005). American
Public Health Association (APHA), the American Water Works Association (AWWA),
and the Water Environment Association (WEF).